February 26. Sponge spicules

Perhaps you recall on February 4 we talked about archaeocyathids, animals that most scientists believe were an early type of sponge. By the Late Cambrian, the archaeocyathids were extinct, and that might be because of the increase in numbers and diversity of more modern sponges.

There were probably sponges in late Precambrian time. The Ediacara fauna includes probable sponges. But these primitive animals took part in the Cambrian explosion, specifically the explosion in development of hard parts – the same kinds of hard parts that sponges have today.

Most modern kitchen sponges are plastics made from oil and natural gas, but natural sponges are still harvested for household sponges. Their roundish or cylindrical bodies consist of spongin, a protein similar to the collagen in humans that makes up things like tendons and skin. And sponges are full of holes – pores, which gives the name to their phylum, Porifera. Those holes are vital to their simple lives, necessary for circulating water to bring in nutrients and wash out wastes. Sponges don’t have nervous systems or circulatory systems. The water they live in does it all for them.

Microscopic sponge spicules

So what happened to sponges during the Cambrian explosion? They developed things called spicules – pointed structures, sometimes microscopic and sometimes macroscopic, that they used to help support their spongy bodies and that may have provided at least a bit of defense against predation.

Although sponge spicules can be made of hardened spongin or calcite like most shells, many are siliceous – SiO₂, the same as the mineral quartz. That’s the most common mineral in the earth’s crust and it’s the most common constituent of sand. Because silica is resistant, sometimes sponge spicules are all that survives in the fossil record from what may have been a great abundance of sponges.

In addition to providing a support structure for sponges, spicules might have served as little fiber optic bars focusing light into a sponge. This might have helped attract algae or other organisms that sponges had symbiotic relationships with, but study of this aspect of sponges is pretty new. It does have implications for the fiber optic industry, because the cold-temperature secretion of silica by sponges would probably be cheaper than the high temperatures used in the industry today, and it might allow for more efficient introduction of impurities to improve optical characteristics. A common example of such impurities being used is the photo-sensitive chemicals introduced into eyeglasses to make them darken in ultraviolet light. Another is adding tiny amounts of the rare-earth element lanthanum to improve the refractive properties of camera lenses. I discuss some of these applications in my other book, What Things Are Made Of and if you’re interested, you can find information about it here.

Today is Joseph Le Conte’s birthday. He was born in 1833 on Woodmanston Plantation, Georgia, and he became a prominent physician and geologist, the first professor of geology at the University of California at Berkeley. He was a friend of John Muir and was a founding member of the Sierra Club.

—Richard I. Gibson

Photo by NOAA (public domain).